866 Handbook of Nature-Study 



rain, because the force of gravity, which acts on everything on the earth's 

 surface and on the air as well, would soon settle all differences and the at- 

 mosphere would become perfectly still. But the earth is composed of land 

 and water and the land heats up more rapidly under sunshine than the 

 water and also gives off — "radiates" its heal more rapidly than water. As 

 a result, the air over the land is warmer in summer than the air over the 

 water. During the winter this is reversed, and the air over the oceans is 

 warmer than the air over the land. The great ocean currents, by carrying 

 the heat from the equatorial regions toward the poles, and by bringing the 

 cold from the polar regions toward the equator, assist in maintaining a con- 

 stant difference in temperature between the continents and the adjacent 

 oceans. 



Furthermore, the fact that the path of the earth about the sun is not a 

 circle but an ellipse, and that the axis of the earth is not perpendicular to 

 the plane of its orbit, result in an unequal distribution of heat over the sur- 

 face. It is always warmer near the equator than at the poles, and warmer 

 in summer than in winter. All these differences in temperature cause 

 corresponding differences in density, which, in turn, cause differences in 

 weight or pressure over various parts of the earth's surface. These changes 

 are, in no way, the result of chance but are determined by the operation of 

 fixed natural laws, and with this in mind we may now take up the study of 

 the winds of the world. 



The Winds of the World 



The general circulation of the atmosphere may 

 be best studied by disregarding those smaller differ- 

 ences of temperature and pressure that result from 

 local causes and by viewing the earth and its atmos- 

 phere as a whole, considering only those larger 

 differences which are in constant operation. In 

 the great oceans of the world we find the water con- 

 stantly moving in a very systematic manner, and 

 we call this system of movements ocean currents. 

 The Gulf Stream, the Equatorial Current,the Japan 

 Current and others may be hkened to great rivers 

 Snow crystal. of water moving systematically on their courses 



Photomicrograph by in the occan. 

 w. A. Bentiey. Thcrc are greater rivers of air in the atmosphere 



than any in the oceans, and they move on their 

 courses with equally systematic precision and in obedience to fixed laws, 

 which we may in a measure understand. 



The river, at the bottom of which we live, is broad and deep, extending 

 in width from Florida northward nearly to the north pole. It flows from 

 west to east circling the globe and its name is The Prevailing WesterHes. 

 The other river in this hemisphere extends southward from latitude about 

 35° nearly to the equator. Its name is The Northeast Trade Winds. 



In the southern hemisphere are two similar rivers, one extending south- 

 ward from latitude about 30° nearly to the south pole with its current, 

 like its counterpart in the northern hemisphere, flowing from west to 

 east, circling the globe. It is also called The Prevailing Westerlies. 

 The other river in the southern hemisphere extends from about latitude 

 30° northward nearly to the equator and flows from the southeast 



